The present invention relates to laminate substrates and more particularly to laminate substrates for use in the manufacturing of Ball Grid Array (BGA) electronic modules.
A recent development of technology has introduced the use of Printed Circuit Board (PCB) laminates as substrates for the manufacturing of electronic modules which can be of the Single Chip Module (SCM) type or Multi Chip Module (MCM) type. These modules are provided with a plurality of conductive pads for electrical connection with electronic circuits (such as mother boards, back planes, application boards). The electrical connection is achieved by little spherical portions of solder alloy which give the name of Ball Grid Array (BGA) to this kind of electronic module. Usually such modules use PCB laminates made of organic material. These modules are usually called Plastic Ball Grid Arrays. The definition "Plastic" indicates the organic nature of the PCB as opposed to a ceramic substrate. Another example of a BGA module is the Tape BGA (TBGA) which uses a tape of organic material as substrate instead of the laminate.
FIG. 1 is an example of a section of a BGA module of the SCM type. On the lower face of the laminate 101 there is a plurality of conductive pads 103, each pad provided with a solder ball 105 which will be put in contact with an electronic circuit and reflowed, thereby realizing the electrical connection. On the upper face of the module there is the active element 107 covered by a resin cap 109 which protects the active element. In the manufacturing of electronic modules it is common to provide the substrates with plated via holes to electrically interconnect the different conductive layers of the substrate. The substrate can be of the NIP (No Internal Plane) type, having only two conductive layers positioned on the external faces of the substrate, or can be a multi-layer substrate, having also one or more intermediate conductive layers.
An alternative PBGA module is constituted by the Cavity Down package as shown in FIG. 2. The main difference of the Cavity Down module (as opposed to the Chip-Up module described above with reference to FIG. 1) is that the active element 207 is attached on the lower side of the module, on the same side of the solder balls 205 (with conductive pads 203) and it is positioned in a sort of cavity of the organic substrate 201, which completely surrounds the active element 207. This arrangement presents some advantages with respect to the Chip-Up PBGA module. One of the advantages is the reduced thickness of the resulting package, since the chip is "contained" in the substrate. Furthermore, these modules provide a better heat dissipation, because the active element is usually attached to a metal stiffener which constitutes the top face of the module and also acts as heat dissipator.
The BGA technology has a number of advantages over traditional technologies such as the Pin Grid Arrays in terms, for example, of reliability, robustness and cost of manufacturing. Within the BGA technology the Plastic BGA represent a much cheaper solution than using other substrates, like the ceramic ones. However the state of the art Plastic BGA modules are not suitable for every kind of application.
An example is the use of PBGA in high-frequency applications. This kind of device requires Electro Magnetic Interference (EMI) shielding to avoid background interference with the product working frequency. The higher the frequency is, the shorter the related wave length. If the wave length is short enough, it can pass through the atomic structures between molecule and molecule and the signal can cross the materials commonly used for the manufacturing of the electronic packages. If this happens, the interferences/disturbances can reach the active circuit on the chip, couple with correct working signals, latching or delatching circuits in a completely arbitrary way, causing the chip functions to be unrecognisable or unusable and, in some cases, even physically damaging the application. In a normal environment, there are several possibilities for random signals with HF/Radio Frequency characteristics, such as electrical spikes, household disturbances, short wave rays (X-Rays) present in the atmosphere and many others. To avoid this major problem, it is necessary to protect the RF application with a kind of box made with a material having a very tight molecular structure, such as a metal, that cannot be crossed by the RF interferences but which reflects them back. This metal box operates as a Faraday cage that protects the application functionality.
It is known to manufacture devices for HF applications using an all metal cavity package to house the electronic circuit. In hybrid microelectronic circuits the substrate (usually ceramic) bearing silicon dice and passives is glued or brazed to bottom of metal package. Then wire bonding interconnections are formed between substrate and package leads. Then the whole module is capped by brazing or welding a cover lid to the open cavity in order to get a one piece all metal package. The EMI shielding is thus obtained by grounding the metal envelope through internal interconnections.
However, this solution involves considerably high costs. Use of low cost organic packages would be very desirable, but the state of the art PBGA do not meet the requirement of EMI Shielding.